Water guns which operate under air pressure are known, such as the device described in U.S. Pat. No. 5,074,437. Such devices include a water reservoir which can be pressurized by actuation of a piston pump to enable water to be ejected from the nozzle of the water gun as a result of the pressure developed within the water reservoir. It is an object of the present invention to provide an attachment for such water guns which can convert the guns to a device which can eject projectiles such as foam balls.